A start methionine mutation in matriptase causes the membrane anchor to be skipped and replaced by a cryptic signal peptide.

The serine protease matriptase is expressed on the plasma membrane of many cell types. Matriptase knockout mice die shortly after birth, whereas in humans, homozygosity for a start-loss mutation (matriptase M1I) is compatible with life. Because the remaining in-frame methionine codons are located downstream of the transmembrane domain, matriptase M1I is predicted to lack its membrane anchor and, at best, become nonfunctional due to mislocalization to the cytosol. This study aims to determine whether matriptase M1I is produced as a correctly located potentially functional protein. Our findings show that matriptase M1I can be expressed in a cell model as a truncated protein detectable by antibodies against matriptase. Using cell-surface biotinylation and streptavidin purification, we demonstrate that matriptase M1I is present on the plasma membrane and exhibits matriptase-specific enzymatic activity. Bioinformatic analyses predict the presence of a non-cleavable cryptic signal peptide downstream of the methionine residue at position 180 (wild-type matriptase numbering), resulting in the same membrane topology as wild-type matriptase. Initiation of translation from this alternative start codon is predicted to generate a membrane-bound protein lacking the cytosolic tail, the transmembrane domain, and the SEA domain of wild-type matriptase, while gaining a non-cleavable signal peptide derived from a sequence that normally forms part of the SEA domain in wild-type matriptase. Thus, it seems likely that the human genetic variant matriptase M1I is expressed as an enzymatic active protein with the correct subcellular localization and thus may substitute for the essential function of wild-type matriptase.